This invention relates generally to liquefied natural gas (LNG) systems. More specifically, it relates to arrangements for condensing boil-off gas from an LNG storage tank and condensing the gas into a send-out stream.
LNG is stored in storage tanks throughout the world. It is typically stored in liquid form at low pressure and cold temperatures, and is pumped to a high pressure before being vaporized and sent out through a natural gas pipeline or distribution system. The pumping operation typically involves a set of low-stage pumps (usually located in the storage tank) that pump the liquid to an inter-stage pressure and a set of high-stage booster pumps (located outside the storage tank).
Boil-off gas (BOG) is the result of heat input into a storage tank that causes some of the stored liquid to vaporize. It can also include vapor displaced by liquid when the tank is filled. Boil-off gas can also be generated from an outside source such as a ship. The BOG is generally at low pressure. Several means are available for disposing of the low pressure BOG. It can be vented to atmosphere or flared, although both of these methods are environmentally unfriendly. In addition, these methods waste a valuable commodity.
Preferably, the BOG is routed to the distribution system or pipeline. High pressure, high horsepower compressors would be required to compress the BOG to pipeline pressure, which can be as high as 100 bar. Condensing the BOG into a liquid send-out stream is more efficient.
Several existing LNG import terminals use systems in which the cold LNG send-out is used to condense BOG at an inter-stage pressure. The BOG condensate can then be combined with the liquid send-out flow before it reaches the booster pumps. Granger's U.S. Pat. No. 6,745,576, for example, discloses a system for re-condensing BOG using a mixing device and a separating device. Engdahl's U.S. Pat. No. 6,470,706 discloses a packed-bed condensing system for re-incorporating BOG into a liquid send-out stream. In the arrangement disclosed in FIG. 1 of that patent, all the liquid send-out from the storage tank passes through the BOG condenser, part as condensing fluid and part as pump fluid. The condensing fluid is used to condense compressed BOG, and the condensate is mixed with the pump fluid before being sent to the booster pumps. In other prior art systems, such as the Dabhol Power Company LNG terminal in Dabhol, India, only a portion of the liquid send-out is routed to the condenser. The condensate is returned to the liquid send-out line through a condensate line.
In the arrangement disclosed in the '706 patent, the flow of condensing fluid is varied to try to maintain a relatively constant pressure in the condenser. If pressure increases, the flow of condensing fluid is increased. If increasing the flow of condensing liquid is not sufficient to keep the pressure in the condenser within the desired range, BOG is vented to a flare. If pressure in the condenser decreases, the flow of condensing fluid is decreased. If decreasing the flow of condensing fluid is not sufficient to keep the pressure within the desired range, make-up gas is introduced into the condenser.
The Dabhol facility similarly provides for venting BOG or adding make-up gas if the pressure in the condenser departs from a desired range. The liquid level in the condenser is generally controlled by adjusting the position of two level control valves on a segment of the liquid send-out line that bypasses the condenser and operates in conjunction with an additional valve and flow meter on the condensing liquid line.
In existing systems, pressure fluctuations in the condenser vessel can significantly affect the operation of the booster pumps and general operation of the pump-out and vaporization system. Also, if the BOG condensing system is not operational, the critical pump-out and vaporization system is not operational.
It is believed that an even better arrangement is possible.